VFD-Motor Speed and Torque

Sunday, January 10, 2016

Question: I have a 1.5 kW three phase motor running on a GoHz VFD. The VFD is connected to two phase / 220V supply, the motor is connected in Delta connection. RPM of the motor is 2800 / 3360 at 50 / 60 Hz. It is now running on 50 Hz. I need a RPM of 1400 to 1500 which means I will need to run the motor on 25 Hz which is not a problem with the GoHz VFD. But how much does it affect the motor torque? Or would it cause any other problem?

Answer: You would probably be fine for torque as the GoHz drive can be programmed to put out full current across a wide speed range. It also has a number of operating modes to play with as well as some parameters to improve torque/control such as boost. If there is an issue it would be cooling for the motor. External cooling may be required depending on load, duty cycle and environment.

On a VFD, frequency is speed, but current is torque. 100% rated current is essentially 100% rated torque regardless of the speed. However, 100% rated current means 100% rated heat, but now with only half the cooling air from the shaft-mounted cooling fan. Your motor may (or may not) be rated for this. Check with the manufacturer. A 2:1 constant-torque duty rating (in your case, 25-50Hz speed range) is not unusual for inverter-duty motors, and if you're not using an inverter-duty motor, you should be. If (as others have also noted) your load does not require 100% torque at reduced speed, heating may not be an issue.

However, your motor is extremely high-slip (200 RPM of slip at 50Hz, or 6.7%. This is pretty unusual, and characteristic of a NEMA Design D motor. Is this a small punch-press by any chance? Something with a large flywheel that operates a cyclic load?

Also check if the motor has star and delta windings you might be able to change the motor connection in the terminal box. To achieve the desired power and current for the given speed. The motor nameplate will tell you if you can. Then you can set the VFD accordingly.